1. Field of the Invention
The present invention relates to an optical sheet capable of changing the travel direction of light. The present invention also relates to a surface light source device and a transmission type display device, each having such a useful optical sheet.
2. Description of Related Art
A surface light source device, which is used in a transmission type display device such as a liquid crystal display device, has a light source and a number of optical sheets (optical films) for changing the travel direction of light from the light source, as disclosed e.g. in U.S. Pat. No. 5,771,328 and JP 8-304608A.
Among the number of optical sheets, in general, an optical sheet having a function (light diffusing function) to diffuse light from the light source and thereby shade (obscure) the image of the light source, an optical sheet having a function (light condensing function) to change the travel direction of light so that the angle (exit angle) between the direction of the exiting light and the front direction becomes smaller, thereby enhancing the front direction luminance, etc. are included.
As the optical sheets having a light condensing function, the optical sheets, having unit prisms (unit shaped elements, unit optical elements) extending linearly and arranged in a direction perpendicular to the longitudinal direction of the prisms (this arrangement manner is the so-called “linear arrangement”), are widely used. The unit prisms of the optical sheet each typically have a triangular, ellipsoidal or circular cross-sectional shape. Such unit prisms thus have ridge lines extending in the longitudinal direction thereof.
Various problems may arise when an optical sheet having unit prisms comes into contact with another member, such as another optical sheet or the light incident side surface of a transmission type display unit such as a liquid crystal display panel. Specifically, a fringe like pattern may be produced in a region where the unit prisms of an optical sheet are in contact with another member. Examples of the fringe like pattern include an annular pattern like so-called “Newton's rings”, a rainbow pattern with rainbow-like color hues, a wetting pattern (also called “wet-out”) observed as staining with a liquid, etc. In conventional practice, a light diffusing sheet is interposed between the prism surface of an optical sheet and a liquid crystal panel so as to make such a fringe like pattern (fringe pattern) invisible.
On the other hand, with a view to producing thinner display devices and reducing the production cost, there is a strong demand in these days for reduction of the number of optical sheets incorporated in a surface light source device (display device). In some display devices, therefore, a light diffusing sheet is not provided between an optical sheet having prisms and a liquid display panel, and the unit prisms of the optical sheet are in contact with the smooth light incident side surface of the transmission type display unit. The above-described fringe like pattern is likely to be clearly visible in such a display device compared to the conventional common display device having a light diffusing sheet disposed between a transmission type display unit and an optical sheet having unit prisms.
Thus, in order to meet the demands for thinner display devices and reduced production cost, a strong desire exists to prevent the formation of a fringe like pattern by devising an appropriate construction of an optical sheet having unit shaped elements without using an additional light diffusing sheet.
In order to attain this aim, U.S. Pat. No. 5,771,328 discloses an optical sheet in which part of the unit prisms (unit shaped elements) is made higher than the other prisms. According to the optical sheet of U.S. Pat. No. 5,771,328, only the higher unit prisms make contact with another member. The patent is thus intended to decrease the contact area between the optical sheet and another member and to position most of the unit prisms at a distance from a display unit (display panel), thereby reducing problems due to the contact. However, when bend or deformation occurs in the optical sheet of U.S. Pat. No. 5,771,328, the contact area of the optical sheet with an adjacent member can become considerably large, and this such a bend or deformation makes it impossible to sufficiently obscure a fringe like pattern. Furthermore, the production of the optical sheet disclosed in U.S. Pat. No. 5,771,328 is difficult because of the complicated shape of prisms, and this difficulty in the production makes it also difficult to lower the production cost.
With reference to the optical sheet disclosed in JP 8-304608A, the contour of the top of each unit prism (unit shaped element) undulates continuously and moderately in the ridge line direction. Accordingly, the optical sheet makes contact with another member only in the locally high portions of the unit prisms. The techniques disclosed in JP 8-304608A are to decrease the contact area between the optical sheet and another member and to position most of the ridge portion of each unit prism at a distance from a display unit, thereby reducing problems due to the contact. However, it is technically difficult to sharply change the height of each unit prism in an optical sheet as disclosed in JP 8-304608A. Thus, the change in the height of the unit prisms of the optical sheet mentioned in JP 8-304608A should practically be quite moderate, and this moderate change makes it impossible to achieve a sufficiently small contact are between the optical sheet and another member. Further, the contact area between the optical sheet of JP 8-304608A and an adjacent member will become considerably large when the contact pressure between them increases due to deformation of the optical sheet caused e.g. by moisture absorption or thermal expansion. Thus, the optical sheet disclosed in JP 8-304608A also sometimes cannot sufficiently obscure a fringe like pattern. In addition, the production of the optical sheet disclosed in JP 8-304608A is difficult, and this difficulty in the production makes it also difficult to lower the production cost.